Dynamic photoswitching of helical inversion in liquid crystals containing photoresponsive axially chiral dopants

Article abstract of DOI:10.1021/ja2088053

Chirality switching is intriguing for the dynamic control of the electronic and optical properties in nanoscale materials. The ability to photochemically switch the chirality in liquid crystals (LCs) is especially attractive given their potential applications in electro-optic displays, optical data storage, and the asymmetric synthesis of organic molecules and polymers. Here, we present a dynamic photoswitching of the helical inversion in chiral nematic LCs (N*-LCs) that contain photoresponsive axially chiral dopants. Novel photoresponsive chiral dithienylethene derivatives bearing two axially chiral binaphthyl moieties are synthesized. The dihedral angle of the binaphthyl rings changes via the photoisomerization between the open and closed forms of the dithienylethene moiety. The N*-LCs induced by the dithienylethene derivatives that are used as chiral dopants exhibit reversible photoswitching behaviors, including a helical inversion in the N*-LC and a phase transition between the N*-LC and the nematic LC. The present compounds are the first chiral dopants that induce a helical inversion in N*-LC via the photoisomerization between open and closed forms of the dithienylethene moiety.

Full text of DOI:10.1021/ja2088053

Article
pubs.acs.org/JACS
Dynamic Photoswitching of Helical Inversion in Liquid Crystals
Containing Photoresponsive Axially Chiral Dopants
Hiroyuki Hayasaka,† Tatsuaki Miyashita,† Masaru Nakayama,‡ Kenji Kuwada,† and Kazuo Akagi*,†
†Department of Polymer Chemistry, Kyoto University, Katsura, Kyoto 615-8510, Japan
‡Institute of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan
S
* Supporting Information
ABSTRACT: Chirality switching is intriguing for the dynamic
control of the electronic and optical properties in nanoscale
materials. The ability to photochemically switch the chirality in
liquid crystals (LCs) is especially attractive given their potential
applications in electro-optic displays, optical data storage, and the
asymmetric synthesis of organic molecules and polymers. Here,
we present a dynamic photoswitching of the helical inversion in
chiral nematic LCs (N*-LCs) that contain photoresponsive axially
chiral dopants. Novel photoresponsive chiral dithienylethene
derivatives bearing two axially chiral binaphthyl moieties are
synthesized. The dihedral angle of the binaphthyl rings changes
via the photoisomerization between the open and closed forms of the dithienylethene moiety. The N*-LCs induced by the
dithienylethene derivatives that are used as chiral dopants exhibit reversible photoswitching behaviors, including a helical
inversion in the N*-LC and a phase transition between the N*-LC and the nematic LC. The present compounds are the first
chiral dopants that induce a helical inversion in N*-LC via the photoisomerization between open and closed forms of the
dithienylethene moiety.
1. INTRODUCTION
Dynamic photoswitching of the chiral structure and the
chiroptical properties in a chiral nematic LC (N*-LC) by
means of external light is useful not only for the development
of molecular devices and optical data storage systems1−5 but
also for the construction of an asymmetric reaction field that
enables the synthesis of helical polymers.6−9 The chiral dopants
employed here are either chiral-center-containing compounds
or axially chiral compounds. Axially chiral binaphthyl derivatives
are feasible for controlling the helical sense and strength of the
N*-LC, especially when their molecular structures have been
chemically modified, such as by the introduction of long alkyl
groups or mesogenic substituents onto the naphthyl rings.10
The binaphthyl derivative is known to have a restricted
freedom of internal rotation along the carbon−carbon bond
between the 1 and 1′ positions of the binaphthyl rings. The
helicity of the binaphthyl derivative therefore depends on the
dihedral angle (θ) between the two naphthyl rings. For
example, the R-binaphthyl derivative of the cisoid conformation
(0° < θ < 90°) and that of transoid conformation (90° < θ <
180°) have (M)- and (P)-helicity, respectively, as described in
Figure 1.11 The binaphthyl derivatives of (M)- and (P)-helicity
induce N*-LCs with left-handed (counterclockwise) and right-
handed (clockwise) screw senses (helical senses), respectively,
despite having the same (R) configuration, when they are added as
chiral dopants into N-LCs.10 For example, a nonbridged structure
of the binaphthyl derivative exhibits a transoid conformation, and a
bridged structure exhibits a cisoid conformation, in which the two
Figure 1. Structures of cisoid and transoid conformations of the
binaphthyl derivatives with (R)-configuration.
oxygen atoms at the 2,2′-positions of the binaphthyl rings are linked
with an alkyl spacer. The former conformer results in a N*-LC with
a right-handed screw sense, and the latter conformer produces a
Received: September 19, 2011
Published: January 31, 2012
© 2012 American Chemical Society
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N*-LC with a left-handed screw sense.6e,10d Another example is
that the mono- and disubstituted crown-ether-type binaphthyl
derivatives with the same (R)-configurations give opposite screw
senses to each other in N-LCs, although both of them exhibit
bridged structures.6b These experimental facts suggest that the
chirality of the binaphthyl derivative depends on the local structure
around the 2,2′-positions of the binaphthyl rings, such as a bridged
structure, as well as on the twisting direction and degree of the
dihedral angle defined by the binaphthyl rings. The N*-LCs that
contain axially chiral binaphthyl derivatives have been attracting
current interest because they are useful for asymmetric reaction
solvents that can provide helical conjugated polymers using achiral
monomers.6,7 If the twisting direction and/or the degree of
dihedral angle in the binaphthyl derivative can be changed by
photochemical perturbation, the helical twisting direction and
strength of the N*-LC are also controlled. This may lead to an
advanced stage of asymmetric polymerization that enables us to
dynamically control helical sense and strength of the polymers and
even reaction products.6d
binaphthyl moieties (blue frames) linked with the alkyl
groups in the terminal units. We focus on the control of the
dihedral angle between two naphthyl rings through the con-
formational changes of the dithienylethene moiety. Using
the present photoresponsive chiral binaphthyl derivatives as
a chiral dopant to be added into the N-LC, we achieve
dynamic photoswitching in the helical sense of the induced
N*-LC.
2. RESULTS AND DISCUSSION
2.1. Synthesis and Characterization. The synthetic routes
are shown in Scheme 2. Binaphthyl derivatives, (R)-1−(R)-5 and
Scheme 2. Synthetic Routes and Photoisomerization of the
Photoresponsive Chiral Derivatives
It is well-known that azobenzene, flugide, and spiropyran are
photochromic molecules showing reversible changes in molecular
structure and absorption spectra upon photo- and thermal
induction.8 The chiral compound including azobenzene has
been used as a switching chiral dopant due to its photochemical
trans−cis isomerization.3,4 However, the N*-LC containing the
chiral azobenzene derivative is not feasible for the asymmetric
solvent. This is because the azo group is less fatigue resistant for
the photoisomerization and thermally unstable and furthermore
chemically unstable to active transition metal catalysts, despite
having photoresponsible function and ease of synthesis.
Therefore, it is desired for the N*-LC to have not only an
external force-responsive function with fatigue resistance but
also chemical and thermal stability, when the N*-LC is used as
the photochemically controlled asymmetric reaction field.6h,i
The dithienylethene derivative is one of the most promis-
ing photoresponsive compounds because of its outstanding
fatigue resistance, thermal stability, and ability to undergo
conformational changes between the open and closed forms
via photoisomerization.1c,12
Here, we synthesize the novel axially chiral binaphthyl deriva-
tive bearing the dithienylethene moiety at the binaphthyl ring
and use them as chiral dopants to induce N*-LCs. The N*-LCs
exhibit the reversible photoswitching behavior in helical
inversion and strength and phase transition. We demonstrate
that the helical sense and the helical pitch of the N*-LCs are
well controlled and even tuned by changing the length of the
alkyl group substituted at the 2-position of the binaphthyl
ring. The photoresponsive axially chiral binaphthyl deriva-
tives, (R)-D1−(R)-D5 and (S)-D1, are described in Scheme 1.
Scheme 1. Structures of Chiral Binaphthyl Derivatives
Bearing Photoresponsive Dithienylethene Moieties
They are composed of the photoresponsive dithienylethene
moiety (red frame) in the central unit and the axially chiral
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(S)-1, were synthesized through alkaline-mediated coupling
between chiroptical (R)-(+)- or (S)-(−)-1,1′-bi-2-naphthol and
alkyl halides in acetone. The dithienylethene derivative 6, which
had been synthesized from 5-methylthiophene-2-carboxyalde-
hyde with five steps,13,14 was coupled with (R)-1−(R)-5 and
(S)-1 via Mitsunobu reaction in THF,15 giving the photo-
responsive chiral binaphthyl derivatives, (R)-D1−(R)-D5 or
(S)-D1, respectively.
The photoisomerization of the dithienylethene moiety upon
1
irradiation with UV and visible (vis) light was examined by H
NMR measurements in CDCl3. The signal observed at approxi-
mately 5.00 ppm was assigned to the methylene proton that
neighbors the dithienylethene moiety with the open form.
Irradiation with UV light (λ = 254 nm, 16 W) induced the
photoisomerization and resulted in a new signal at approx-
imately 4.70 ppm that corresponded to the dithienylethene
moiety with the closed form. The integrated intensity of the
proton signal indicated that 30% of the open form dithienyl-
ethene moiety was converted to the closed form in the photo-
stationary state (PSS) of (R)-D1. The degree of conversion to
the PSS (closed form) is determined by the following formula:16
{[closed form]/([open form] + [closed form])} × 100. This
formula was used to determine that 30% was converted to the
closed form, and that the remaining 70% maintained its open
form, even after irradiation with UV light. (See Figures S1−S3
in the Supporting Information.) After the subsequent irradia-
tion of the sample with vis light (λ > 400 nm, 100 W), the
proton signal of the open form reappeared. Similar degrees of
conversion in PSS were observed in (R)-D2−(R)-D5 and (S)-D1.
Because the closed form is not completely formed even after
irradiation with UV light, the expression “PSS” will be used
hereafter instead of “closed form”.
Figure 2. (a) Changes in UV−vis absorption and (b) CD spectra of
(R)-D1 in n-heptane (1.5 × 10−5 M) upon irradiation with UV light
(λ = 254 nm, 16 W) for 0, 5, 10, and 15 s at 30 °C. Inset shows the
expanded spectra. (c) Difference spectra given by subtraction of the
CD spectrum of (R)-D1 (open form) from those of (R)-D1 (PSS)
upon irradiation with UV light for 0, 5, 10, and 15 s.
2.2. Absorption and Circular Dichroism. (R)-D1 (open
form) dissolved in isotropic n-heptane (1.5 × 10−5 M) showed
absorption bands that corresponded to the polarized naphthyl
1
and Lb (300−350 nm) transitions that correspond to the
binaphthyl moieties.11,18 (See also Figures S4−S6 in the
Supporting Information.)
1
moiety transition moments of the Bb long-axis at approx-
imately 200−250 nm, the 1La short-axis at approximately 250−
Interestingly, when the dithienylethene moiety of (R)-D1
was photoisomerized from the open form to the closed form
upon irradiation with UV light for 15 s, the peaks at 223 and
237 nm that are attributed to the 1Bb transitions of the
binaphthyl moieties gradually increased. In addition, the peaks
decreased in intensity after irradiation with visible light for 60 s.
Figure 2c shows the different CD spectra for (R)-D1 (open
form) and (R)-D1 (PSS), which clearly indicate an increase in
Δε of the transition of the binaphthyl rings. Here, Δε is defined
as εL − εR, where εL and εR are the molar excitation coefficients
of left and right circularly polarized lights, respectively. The
change in Δε between (R)-D1 (open form) and (R)-D1 (PSS),
|Δεopen form − ΔεPSS|, at 233 nm was 80. Rosini et al. have
reported that the Δε of the 1Bb couplet at 230 nm is correlated
with the dihedral angle of the binaphthyl moieties.11f On the
basis of this correlation, the dihedral angles of (R)-D1 (open
form) and (R)-D1 (PSS) were approximately 100° and 90°,
respectively. That is, the binaphthyl moieties of (R)-D1 (open
form) and (R)-D1 (PSS) have a transoid conformation [(P)-
helicity] and an orthogonal conformation, respectively, in
n-heptane. This result suggests that the conformational change
of the dithienylethene moiety via photoisomerization in the
local space around the 2-position of the binaphthyl moieties
causes a change in the dihedral angle between the two naphthyl
rings. The change in the dihedral angle of the binaphthyl rings
upon the photoisomerization of the dithienylethene moiety
would allow us to control the helical sense and/or the helical
1
300 nm, and the Lb long-axis at approximately 300−350 nm.
After being irradiated with UV light for 15 s, the shoulder band
at approximately 255 nm that is attributed to the open form of
the dithienylethene moiety decreased in intensity. At the same
time, the bands at approximately 340 and 515 nm that are
attributed to the closed form of the dithienylethene moiety
gradually increased in intensity, as shown in Figure 2a. Sub-
sequent irradiation of vis light for 60 s after the irradiation with
UV light caused an increase in the intensity for the band at
approximately 255 nm and a decrease in the bands at approxi-
mately 340 and 515 nm. This means that the dithienylethene
moiety returned to its original state.
The photoresponsive chiral compounds, (R)-D1−(R)-D5
and (S)-D1, showed circular dichroism (CD) signals in the
region of the absorption spectra. Note that the closed form of
the dithienylethene derivative is known to have the same amount
of (R,R) and (S,S) enantiomers of the thiophene rings.17
(See Scheme S1 in the Supporting Information.) In fact, no
Cotton effect was observed in the visible region centered
around 550 nm corresponding to the absorption band of the
closed form, indicating no occurrence of the diastereose-
lective photocyclization in the present systems. As typically
shown in Figure 2b, (R)-D1−(R)-D5 with (R)-binaphthyl
derivatives exhibited CD signals, which displayed large posi-
1
tive and negative Cotton effects for the Bb (200−250 nm)
transitions and positive Cotton effects for the 1La (250−300 nm)
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pitch of the N*-LCs induced by addition of the present
binaphthyl derivatives by using alternate irradiations with UV
and vis light.
the construction of dynamically controllable asymmetric reaction
field because of its chemical stability.6a,b
The N*-LC induced by adding (R)-D1 as a chiral dopant to
PCH or 5CB showed a finger-printed texture in the polarized
optical microscope (POM). Similar finger-printed textures were
also observed in the N*-LCs induced by (R)-D2−(R)-D5 and
(S)-D1. The temperature ranges of the N* phase for PCH and
5CB were 7−31 °C and 13−30 °C, respectively, in the heating
process and 0−30 °C and −5−29 °C, respectively, in the
cooling process. The N*-LC that was induced by the addition
of (R)-D1 (open form) into the PCH-type LC showed the
spiral and fingerprinted texture in POM as shown in Figure 4a.
After the sample was irradiated with UV light for 30 s at 30 °C,
the texture notably changed; first, the fingerprinted texture
gradually changed into the Schlieren texture that is char-
acteristic of a nematic phase. Next, the fingerprinted texture
with an opposite screw sense reappeared. However, the irradia-
tion of the N*-LC that included (R)-D1 (PSS) with vis light
(λ > 400 nm, 100 W) for 2 min at 30 °C gave rise to the
reverse in POM.
The change in the dihedral angle of the binaphthyl rings
upon the photoisomerization of the dithienylethene moiety is
supported by the CD spectra of (R)-D1 in n-hexane. The CD
signals due to Cotton effects for the 1Bb and 1La transitions that
corresponded to the binaphthyl moieties of (R)-D1 increased
in intensity upon the irradiation with UV light (Figure 2b and c),
and they decreased in intensity upon the irradiation with vis
light (Figure 3). Similar reversible changes in absorption and
2.4. Screw Sense of N*-LC. The screw sense of the N*-LC
was next examined through the contact test based on the
miscibility between two LC compounds.19 The cholesteryl oleyl
carbonate (COC), known to be left-handed, was used as the
LC standard. This method is based on the observation of the
mixing area between the N*-LC and the LC standard using
POM. When the screw sense of the N*-LC is the same as that
of the COC, the mixing area will be continuous. Otherwise, it
will be discontinuous. In the contact method, the mixture of
N*-LC that included (R)-D1 (open form) in PCHs and the LC
standard of cholesteryl oleyl carbonate (COC) with a left-
handed screw sense gave a discontinuous boundary, as shown
in Figure 5. This discontinuous boundary indicates that the
screw sense of the N*-LC that included (R)-D1 (open form) is
opposite of that of the COC, that is, the right-handed one.
After irradiation with UV light, the mixing area gave a con-
tinuous boundary, which indicates that the screw sense of the
N*-LC that included (R)-D1 (PSS) is the same as that of the
COC; that is, it is left-handed. Subsequently, the irradiation
with vis light for 2 min at 30 °C inverted the original screw
sense from left- to right-handed. These results indicate that the
N*-LCs induced by (R)-D1 (open form) and (R)-D1 (PSS),
which possess the same (R)-configuration, exhibited a drastic
helical inversion under alternating irradiations with UV and
vis light, as was also schematically described in Figure 4a.
Similarly, the N*-LC induced by (S)-D1 in PCH also exhi-
bited the photoinduced helical inversion under alternating
irradiations with UV and vis light. Therefore, it is demon-
strated that accompanying the photoisomerization, the N*-LCs
that include (R)-D1 and (S)-D1 show a reversible helical inver-
sion in the screw sense.
Figure 3. Changes of CD spectra of (R)-D1 and (S)-D1 in n-heptane
(1.5 × 10−5 M) upon alternating irradiations of UV (λ = 254 nm, 16 W)
and vis (λ > 400 nm, 100 W) lights.
CD intensity due to the photoisomerization were observed
in (R)-D2−D5 (see Figures S5 and S6 in the Supporting
Information). Note that (S)-D1 exhibited a mirror image of the
CD spectrum of (R)-D1, giving Cotton effects with opposite signs
to those of (R)-D1, but showed the same change in CD intensity
upon irradiations with UV and vis light as in the case of (R)-D1, as
shown in Figure 3.
2.3. Photoswitching in N*-LCs. N*-LCs were prepared by
adding a small amount (1.0 mol %) of photoresponsive axially
chiral compound [(R)-D1−(R)-D5 and (S)-D1] as a chiral
dopant into two kinds of nematic LCs consisting of phenyl-
cyclohexyl or cyanobiphenyl mesogen, respectively, as shown
in Scheme 3. The former is an equimolar mixture of N-LCs,
Scheme 3. Structures of Nematic Liquid Crystals
Although the mechanism of the reversible helical inversion in
the N*-LC remains unclear, one of the plausible explanations is
as follows. As schematically described in Figure S7 in the
Supporting Information, the open and closed forms (PSS) of
the dithienylethene moiety reversibly change to each other
upon alternating irradiations of UV and vis light. In accompany
with the photochemical isomerization, the binaphthyl moieties
with (R)-configuration interchange between (R)-transoid and
(R)-cisoid conformations. The stereospecific intermolecular
interactions between the binaphthyl moieties of the chiral
dopant and the surrounding nematic LC molecules should play
a role of trigger to determine the screw sense of nematic LCs.
4-(trans-4-n-propylcyclohexyl)ethoxybenzene [PCH302] and
4-(trans-4-n-propylcyclohexyl)butoxybenzene [PCH304], ab-
breviated as PCH, which is chemically stable toward not only
extremely reactive Ziegler−Natta catalyst such as Ti(O-
n-Bu)4−AlEt3 but also other kinds of transition metal (Ni, Pd, Pt)
compounds and exhibits a N-LC phase in the region from 20 to
35 °C. The latter, 4-cyano-4′-pentylbiphenyl, abbreviated as
5CB, shows a N-LC phase in the region from 24 to 35 °C, but
it is reactive to catalytic species such as AlEt3. These results
imply that the N*-LC consisting of PCH is more favorable for
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Figure 4. Polarized optical micrographs (POMs) of photoresponsive N*-LCs including (a) (R)-D1, (b) (R)-D3, and (c) (R)-D5 (1.0 mol %)
between the open form (left) and PSS (right) by irradiation of UV (λ = 254 nm, 16 W, handy lamp) and vis (λ > 400 nm, 100 W) light at 30 °C.
Along the corresponding POMs are described schematic illustrations of (a) reversible helical inversion between N*-LCs with opposite screw senses,
(b) reversible phase transition between N*-LC and N-LC, and (c) reversible change in helical pitch within N*-LC upon alternating irradiations of
UV (left) and vis (right) light.
The chiral dopant consisting of dithienylethene with open form
bearing (R)-transoid binaphthyl moieties induces righted-
handed N*-LC; the chiral dopant consisting of dithienylethene
with closed form bearing (R)-cisoid binaphthyl moieties induces
left-handed N*-LC.
determined using the Grandjean−Cano method and the
droplet method in glycerol,20 respectively. The βM is defined
as βM = 1/(pcr), where p is the helical pitch and c is the molar
concentration of the chiral dopant, and r is the enantiomeric
purity of the chiral dopant. Here, r is assumed to be 1.21
Positive and negative signs represent a right-handed (clock-
wise) and a left-handed (counterclockwise) screw sense, respec-
tively. Table 1 summarizes the helical screw senses and helical
twisting powers (βM) of the photoresponsive chiral com-
pounds, (R)-D1−(R)-D5 and (S)-D1 in PCH and 5CB. The
βM increased with an increase of the length of the alkoxy groups
in the binaphthyl moiety [(R)-D1−(R)-D5], but the increase in
βM decreased the change (ΔβM) in βM between the open form
and PSS of the dithienylethene moiety. The βM’s of (R)-D1
(open form) and (R)-D1 (PSS), which have opposite screw
senses to each other in PCHs, were +6.6 and −8.3 μm−1,
respectively. The ΔβM of (R)-D1 after the photoisomerization
of the dithienylethene moiety in PCH was 14.9 μm−1. The βM’s
of (R)-D2 and (R)-D3 that had undergone photoinduced phase
transitions between N*-LC and N-LC in PCH were 11.7 and
12.7 μm−1, respectively. The ΔβM’s of (R)-D4 and (R)-D5 in
PCH slightly changed by 1.9−2.4 μm−1 after irradiation with
UV and vis light. On the other hand, the N*-LCs prepared by
using 5CB as an N-LC host showed no helical inversion but
only a slight change or no change in helical pitch. The ΔβM’s of
(R)-D1 and (S)-D1 in 5CB changed by 5.6−5.9 μm−1, and
those of (R)-D2 and (R)-D3 in 5CB slightly changed by 0.6−
1.6 μm−1 after irradiation with UV and vis light. There is no
change in ΔβM of (R)-D4 and (R)-D5 in 5CB.
Meanwhile, when both N*-LCs that are induced by (R)-D2
(open form) and (R)-D3 (open form) and that exhibit right-
handed screw senses in PCHs were irradiated with UV light for
30 s, photoinduced phase transitions from the finger-printed
texture (N*-LC) to the Schlieren texture (N-LC) were ob-
served, as shown in Figure 4b. These transitions occurred
because the helical twisting powers (βM) of (R)-D2 (PSS) and
(R)-D3 (PSS) in PCH are insufficient (βM = 0 to 1.0 μm−1)
to induce a stable N*-LC phase. After irradiation of these mix-
tures with vis light for 2 min at 30 °C, the finger-printed tex-
tures with a right-handed screw sense reappeared. The revers-
ible phase transitions between N*-LC and N-LC induced by
(R)-D2 and (R)-D3 in PCHs occurred after irradiation with
UV and vis light.
In the case of the N*-LCs induced by (R)-D4 and (R)-D5 in
PCH, the helical pitch length of the finger-printed textures
increased after irradiation with UV light, as shown in Figure 4c.
Similar behaviors were observed in the N*-LCs induced by
(R)-D1−(R)-D3 and (S)-D1 in 5CB. These results imply that
the dihedral angle between the two naphthyl rings slightly
changed in conjunction with the photoisomerization of the
dithienylethene moiety.
2.5. Changes in Helical Twisting Powers. The helical
pitch and the helical twisting power (βM) of N*-LC were
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Figure 5. Contact tests between the N*-LC including (a) (R)-D1 (open form) or (b) (R)-D1 (PSS of closed form) and the LC standard,
cholesteryl oleyl carbonate with a left-handed screw sense at 30 °C. The N*-LCs are composed of PCH302, PCH304, and (R)-D1 with ratios
of 100:100:2.
It is clear from the above results that the helical sense and
the helical twisting power of N*-LC sensitively depend on the
N-LC host. Actually, there is a notable difference in the behavior
of the helical sense and/or the helical twisting power between
PCH- and 5CB-based N*-LCs upon the photoisomerization of
the chiral dopant. It might be argued that PCH LC molecules
tend to align parallel to the naphthyl ring of the chiral dopant,
and hence they are sensitively affected by the change of the
dihedral angle of the binaphthyl rings (see Figure S7).
Meanwhile, it can be assumed that the 5CB LC molecules
tend to align parallel to the binaphthyl axis of the chiral dopant,
probably due to π−π interactions between the biphenyl moiety
of 5CB molecule and the phenyl−phenyl fragment of the bi-
naphthyl ring of the chiral dopant. Hence, the 5CB LC
molecules are not so drastically affected by the change of
the dihedral angle of the binaphthyl ring as the case of PCH
LC molecules. The above-mentioned arguments, however,
are based on only the initial intermolecular interaction between
the chiral binaphthyl derivative and the LC molecule. The
overall feature including the subsequently occurring arrange-
ment of the rest of LC molecules followed by the construction
of the closest packing structure of the N*-LC needs to be
clarified by further investigations.
Figure 6 shows the changes in the helical twisting powers of
(R)-D1, (R)-D3, and (R)-D5 after alternating irradiations with
UV and vis light. The reversible helical inversion and the
change in helical pitch of N*-LC via the photoisomerization of
the dithienylethene moiety upon irradiation with UV and vis
light repeated for more than 10 cycles. It is worth mentioning
that not only the cycle repeated, but the amplitude of the
change remained consistent. The research to achieve the
asymmetric polymerizations of helical conjugated polymers in
the present N*-LCs is now underway and will be reported
somewhere in the near future.
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and even tuned by changing the length of the alkyl group
substituted at the 2-position of the binaphthyl ring. The present
photoresponsive LCs, especially the N*-LCs that included (R)-
D1−(R)-D3 and (S)-D1, should be applicable for advanced
optical memory devices with multiple read−write functions and
also for helical sense-controllable asymmetric reaction fields.
Table 1. Changes of the Helical Screw Senses and the Helical
Twisting Powers (βM) of the Photoresponsive PCH- and
CB-Based N*-LCs with the Open Form and PSS, with
Positive and Negative Signs Representing Right-Handed
(Clockwise) and Left-Handed (Counterclockwise) Screw
a
Senses, Respectively
helical twisting powers, βM
helical twisting powers, βM
b d
b
c
(μm−1), in PCHs
(μm−1), in 5CB
4. EXPERIMENTAL SECTION
(R)-D1. To a mixture of dithienylethene derivative 6 (140 mg,
0.47 mmol), triphenylphosphine (TPP) (135 mg, 0.51 mmol), and (R)-1
(100 mg, 0.23 mmol) in THF (10 mL), was slowly added a solution of
diisopropyl azodicarboxylate (DIAD) (260 mg, 40 wt % in toluene,
0.51 mmol) in THF (20 mL) via a pressure equalized dropping funnel.
The reaction mixture was stirred at 0 °C overnight under Ar. The
resulting precipitate was extracted with CHCl3, thoroughly washed with
saturated NaCl solution, and dried over anhydrous sodium sulfate. The
precipitate was removed by filtration, and the filtrate was eva-
porated under reduced pressure. The residue was purified via column
chromatography (silica gel; CH2Cl2) to give 122 mg (53%) of
chiral
open
form
|ΔβM
open
form
|ΔβM
e
e
dopant
PSS
(μm−1)|
PSS
(μm−1)|
(R)-D1
(S)-D1
(R)-D2
(R)-D3
(R)-D4
(R)-D5
+6.6
−6.9
+11.7
+12.7
+15.5
+24.3
−8.3
+7.6
1.0
14.9
14.5
11.7
12.7
1.9
−2.2
+2.1
−8.1
+7.7
5.9
5.6
1.6
0.9
0
+6.4
+4.0
1.0
+9.4
+8.3
+13.6
+21.9
+12.3
+20.0
+12.3
+20.0
2.4
0
a
Determined with the miscibility test, in which cholesteryl oleyl
carbonate (COC) is used as the N*-LC standard. Measured with the
Grandjean−Cano method at 30 °C. Composed of PCH302, PCH304,
and chiral dopant with the mole ratio of 100:100:2. Composed of 5CB
and chiral dopant with the mol ratio of 100:1. Difference in helical
twisting power (βM) between the open form and PSS of dithienyl-
ethene moiety.
b
1
c
compound (R)-D1 as colorless powder. H NMR (270 MHz, CDCl3,
d
20 °C, TMS): δ = 1.44 (s, 6H, thienyl−CH3), 3.70 (s, 6H, −OCH3),
4.98 (s, 4H, −OCH2−), 6.59 (s, 2H, thienyl−H), 7.05 (d, 1H, J =
7.74, Ar−H, 8′), 7.15 (ddd, 1H, J = 1.40, 7.54 and 7.74, Ar−H, 7′),
7.16 (d, 1H, J = 8.40, Ar−H, 8), 7.22−7.30 (m, 3H, Ar−H, 6, 6′, and
7), 7.37 (d, 1H, J = 8.90, Ar−H, 3), 7.41 (d, 1H, J = 9.06, Ar−H, 3′),
7.83 (d, 1H, J = 7.58, Ar−H, 5′), 7.86 (d, 1H, J = 9.22, Ar−H, 5), 7.90
(d, 1H, J = 9.06, Ar−H, 4′), 7.96 (d, 1H, J = 8.90, Ar−H, 4). FAB-MS:
m/z: calcd for C59H42O4F6S2, 992.24; found, 992.24 [M+]. Anal. Calcd
for C24H16O3F6S2: C, 71.36; H, 4.26. Found: C, 71.65; H, 4.36.
(R)-D2. The procedure for the synthesis of (R)-D1 was followed to
e
1
prepare (R)-D2 from (R)-2 as a colorless powder in a 25% yield. H
NMR (270 MHz, CDCl3, 20 °C, TMS): δ = 1.01 (t, 6H, J = 6.90,
−OCH2CH3), 1.45 (s, 6H, thienyl−CH3), 4.03 (q, 4H, J = 6.90,
−OCH2CH3), 4.97 (s, 4H, −OCH2−), 6.57 (s, 2H, thienyl−H), 7.08
(d, 1H, J = 6.76, Ar−H, 8′), 7.13 (d, 1H, J = 6.76, Ar−H, 7′), 7.15−
7.35 (m, 4H, Ar−H, 7, 6, 6′, and 8), 7.37 (d, 1H, J = 11.1, Ar−H, 3),
7.41 (d, 1H, J = 11.1, Ar−H, 3′), 7.83 (d, 1H, J = 9.15, Ar−H, 5′), 7.86
(d, 1H, J = 9.15, Ar−H, 5), 7.89 (d, 1H, J = 11.1, Ar−H, 4′), 7.93 (d,
1H, J = 11.1, Ar−H, 4). FAB-MS m/z: calcd for C61H46O4F6S2,
1020.27; found, 1020.27 [M+]. Anal. Calcd for C61H46O4F6S2: C,
71.75; H, 4.54. Found: C, 71.33; H, 5.24.
Figure 6. Reversible changes in helical twisting power (βM) evaluated
with the droplet method using glycerol for (R)-D1, (R)-D3, and (R)-D5
in PCHs upon alternating irradiations of UV (blue area) and vis (yellow
area) lights.
(R)-D3. The procedure for the synthesis of (R)-D1 was followed to
1
prepare (R)-D3 from (R)-3 as a colorless powder in a 24% yield. H
NMR (400 MHz, CDCl3, 20 °C, TMS): δ = 0.52 (t, 6H, J = 7.24,
−CH2CH3), 1.25 (m, 4H, −CH2CH2CH3), 1.44 (s, 6H, thienyl−
CH3), 3.90−3.95 (m, 4H, −OCH2C3H7), 5.00 (m, 4H, −OCH2−),
6.64 (m, 2H, thienyl−H), 6.94 (d, 1H, J = 6.76, Ar−H, 8′), 7.13 (d,
1H, J = 6.76, Ar−H, 7′), 7.15−7.35 (m, 4H, Ar−H, 7, 6, 6′, and 8),
7.37 (d, 1H, J = 11.1, Ar−H, 3), 7.41 (d, 1H, J = 11.0, Ar−H, 3′), 7.83
(d, 1H, J = 9.15, Ar−H, 5′), 7.86 (d, 1H, J = 9.15, Ar−H, 5), 7.89
(d, 1H, J = 11.0, Ar−H, 4′), 7.93 (d, 1H, J = 11.1, Ar−H, 4). FAB-MS
m/z: calcd for C63H50O4F6S2, 1048.31; found, 1048.30 [M+].
3. CONCLUSION
The photoresponsive chiral dithienylethene derivatives [(R)-
D1−(R)-D5 and (S)-D1] bearing two axially chiral binaphthyl
moieties were synthesized and used as chiral dopants to induce
the N*-LCs. The photoisomerization of the dithienylethene
moieties after alternate irradiations with UV and vis light caused
a reversible change in the dihedral angles of the binaphthyl
rings. The photoresponsive binaphthyl derivatives were added
as chiral dopants into N-LCs composed of phenylcyclohexyl or
cyanobiphenyl mesogen, which induced N*-LCs. Accompany-
ing the photoisomerization, the N*-LCs that included (R)-D1
and (S)-D1 showed a reversible helical inversion in the screw
sense. The N*-LCs that included (R)-D2 and (R)-D3 showed a
reversible photoswitching in the phase transitions between N*-
LC and N-LC. The N*-LCs that included (R)-D4 and (R)-D5
showed a notable change in the helical pitch during the
photoisomerization.
(R)-D4. The procedure for the synthesis of (R)-D1 was followed to
1
prepare (R)-D4 from (R)-4 as a colorless powder in a 59% yield. H
NMR (270 MHz, CDCl3, 20 °C, TMS): δ = 0.59 (t, 6H, J = 7.24,
−CH2CH3), 0.98 (m, 4H, −CH2CH2CH3), 1.36 (m, 4H, −CH2C2H5),
1.45 (s, 6H, thienyl−CH3), 3.91−3.97 (m, 4H, −OCH2C3H7), 4.97
(s, 4H, −OCH2−), 6.57 (s, 2H, thienyl−H), 7.08 (d, 1H, J = 6.76,
Ar−H, 8′), 7.13 (d, 1H, J = 6.76, Ar−H, 7′), 7.15−7.35 (m, 4H, Ar−H,
7, 6, 6′, and 8), 7.37 (d, 1H, J = 11.1, Ar−H, 3), 7.41 (d, 1H, J = 11.0,
Ar−H, 3′), 7.83 (d, 1H, J = 9.15, Ar−H, 5′), 7.86 (d, 1H, J = 9.15, Ar−
H, 5), 7.89 (d, 1H, J = 11.0, Ar−H, 4′), 7.93 (d, 1H, J = 11.1, Ar−H,
4). FAB-MS m/z: calcd for C65H55O4F6S2, 1077.34; found, 1077.35
[M + H]. Anal. Calcd for C65H54O4F6S2: C, 72.47; H, 5.05. Found: C,
72.38; H, 5.48.
(R)-D1 and (S)-D1 are the first chiral dopants to induce a
helical inversion in a N*-LC via the photoisomerization between
open and closed forms of the dithienylethene moiety. The
helical sense and the helical pitch of the N*-LCs that included
the photoresponsive binaphthyl derivatives are well controlled
(R)-D5. The procedure for the synthesis of (R)-D1 was followed
to prepare (R)-D5 from (R)-5 as a colorless powder in a 52% yield.
1H NMR (270 MHz, CDCl3, 20 °C, TMS): δ = 0.61 (t, 6H, J =
7.32, −CH2CH3), 0.85−0.98 (m, 8H, −CH2C2H4CH3), 1.38 (q, 4H,
3764
dx.doi.org/10.1021/ja2088053 | J. Am. Chem. Soc. 2012, 134, 3758−3765

Journal of the American Chemical Society
Article
J = 7.32, −CH2C3H7), 1.44 (s, 6H, thienyl−CH3), 3.84−4.00 (m, 4H,
−OCH2C4H9), 4.97 (s, 4H, −OCH2−), 6.57 (s, 2H, thienyl−H), 7.08
(d, 1H, J = 6.76, Ar−H, 8′), 7.13 (d, 1H, J = 6.76, Ar−H, 7′), 7.15−
7.35 (m, 4H, Ar−H, 7, 6, 6′, and 8), 7.37 (d, 1H, J = 9.76, Ar−H, 3),
7.41 (d, 1H, J = 8.54, Ar−H, 3′), 7.83 (d, 1H, J = 8.54, Ar−H, 5′), 7.86
(d, 1H, J = 8.53, Ar−H, 5), 7.89 (d, 1H, J = 8.54, Ar−H, 4′), 7.93 (d,
1H, J = 9.76, Ar−H, 4). FAB-MS m/z: calcd for C67H58O4F6S2,
1104.37; found, 1104.37 [M+]. Anal. Calcd for C67H58O4F6S2: C,
72.81; H, 5.29. Found: C, 71.88; H, 5.40.
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ASSOCIATED CONTENT
* Supporting Information
Details for synthetic procedures of (R)-D1−(R)-D5. H and
■
S
1
13C NMR, UV−vis, and CD spectra of (R)-D1, (R)-D3, and
(R)-D5. Schematic illustrations of reversible helical inversion.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(9) Ishida, Y.; Kai, Y.; Kato, S.; Misawa, A.; Amano, S.; Matsuoka, Y.;
Saigo, K. Angew. Chem., Int. Ed. 2008, 47, 8241−8245.
AUTHOR INFORMATION
Corresponding Author
akagi@fps.polym.kyoto-u.ac.jp
■
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
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This work was supported by a Grant-in-Aid for Science
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